Minimum mesh size limitation for a coaxial via feed in Antenna Toolbox (R2025B)

I am designing a microstrip patch antenna with a coaxial via feed in MATLAB Antenna Toolbox (R2025B).

The inner conductor of the coaxial feed is modeled as an octagon with an equivalent diameter of 1 mm.

When I generate the mesh, I cannot set the maximum mesh edge length to less than 2 mm. If I try to use a smaller value, MATLAB returns an error and the mesh generation fails.

My understanding is that the mesh size should normally be smaller than the diameter of the smallest geometric feature—in this case, the 1 mm inner conductor—to accurately represent the geometry.

As a result, the simulated antenna appears to be poorly matched. The Smith chart does not look physically correct, and the input impedance indicates a significant mismatch that is inconsistent with the antenna geometry.

My questions are:

1. Is this limitation expected behavior in Antenna Toolbox R2025B?
2. Is there an internal restriction that prevents using a mesh size smaller than 2 mm?
3. Could this be a bug in R2025B?
4. Could the inability to use a finer mesh around the coaxial feed be the reason for the incorrect Smith chart and impedance mismatch?

I would appreciate any clarification or suggestions.

Thank you!

Answer 1

[Isha]

Hello,

The exisitng documentation does not describe such a limit and notes that finer meshing is often required near feed locations where current variation is highest.

https://www.mathworks.com/help/antenna/ug/meshing.html

https://www.mathworks.com/help/antenna/ref/cavity.mesh.html

If mesh generation consistently fails for MaxEdgeLength < 2 mm, this may be a geometry-specific meshing limitation or a potential issue in the meshing algorithm. Since the feed region strongly influences the input impedance, an insufficiently resolved mesh could contribute to the unexpected Smith chart and impedance results. https://www.mathworks.com/help/antenna/ug/meshing.html

I recommend contacting MathWorks Technical Support with a reproducible example at https://www.mathworks.com/support/contact_us.html.

Hope this helps.